For example if you have a gaming forum you could start a contest where the user with the highest post count or random user with more than x posts gets a game or something like that.
Content is probably one of the easier solutions; If you do not have a lot of backlinks that bring in traffic.... in todays market, you can find a top forum in just about any type of niche....I have had a forum up for a year now, and all i get is the disrespect of a spammer from time to time...I do the content thing, I post here and there, but that will never be enough if you do not get enough backlinks, and this goes for any website, not just a forum. and yes, i even advertise from time to time, but today, if a forum looks weak, it does not have much of a chance. not to be mean, what do you have that cant be found on DP, Craigslist, SitePoint, etc...I am in the same vote..not much.... There is no quick and easy way to get to riches and success, and most had to work to get what they got. A forum is not a short road to glory...It may even take years to break in the market; chances are, you will give up way before then, most successful forum have been up for years. People should really read up on starting a forum before jumping into the fire...
You can make it better by increasing traffic and this can be done by getting top ranking in search engine results for keywords like High PR Forum site, Goof Forum Site etc.
Try posting some of your articles on "Ezine articles" with a link to your forum in your resource box. You may not get 1000 of hits, but it will generate quality hits that may become regular readers, followers